qt: fix saved interface window states (maximized / fullscreen) are not respected on start

QWindow::show() does not necessarily care about the state of the window, but rather uses the default behavior for the platform.

Since we adjust the window geometry and states before the window is shown (which is the expected behavior, as otherwise the window would get adjusted immediately after shown), using QWindow::show() potentially overrides the window states nullifying the behavior of the interface starting maximized.

I have tested on X11, Wayland, and Windows.

Request review @chub.